Viana will succeed Txiki Begiristain at the end of the season but will begin working with City immediately following the transfer window.
The aim is to ensure a smooth transition, with Viana collaborating closely with Begiristain before officially taking over after the Club World Cup.
A statement released by Sporting on Tuesday morning read: "Sporting Clube de Portugal – Futebol, SAD informs that Hugo Viana requested his departure from the Club to be brought forward, initially scheduled for the end of the current sporting season. Sporting Clube de Portugal – Futebol, SAD further informs that, taking into account the arguments presented and by mutual understanding, the sporting director of Sporting CP ceased his duties after the closure of the winter market.
"Hugo Viana played a fundamental role in the Club's sporting growth during a period in which Sporting CP's main football team won several titles: two National Championships, three League Cups, a Super Cup and a Portuguese Cup.
Sporting CP president Frederico Varandas said: "Hugo Viana's work over these years was fundamental to the implementation of a solid and winning sporting project. He played a crucial role in building a team that broke barriers and marked one of the most memorable phases in the recent history of our Club."